The title translates as 'At the heart of the fantastic', parts of the book have been translated in The Edge of Surrealism.

His definition of the fantastique in this book as "[une] rupture de l'ordre reconnu, irruption de l'inadmissible au sein de l'inaltérable légalité quotidienne". It is most often rendered in English as "The fantastic is always a break in the acknowl­edged order, an irruption of the inadmissible within the changeless everyday legality," as found in Richard Howard's English translation of Tzvetan Todorov's The Fantastic: A Structural Approach to a Literary Genre (1970).

Central to Caillois's definition of the fantastique is the juxtaposition of the real and the unreal, which provokes inquietude ("pour moi, fantastique signifie d'abord inquiétude et rupture"). He wishes to analyze that inquietude by deciphering the "undeciphered indecipherable" ("indéchiffré indéchiffrable").

Among the scholars of the fantastique, he is one of the few to give ample attention to emblem books.

On the 'fantastique naturel'

In writing about the fantastique, Caillois defines it as "always a break in the acknowl­edged order, an irruption of the inadmissible within the changeless everyday legality" (tr. Richard Howard). His definition is often cited.

Interestingly, Caillois invented a second category, the fantastique naturel (the natural fantastic) which seems to contradict his definition. Contradictory is of course that 'everyday legality' is natural so the fantastic would be 'unnatural'. He uses as example two animal species: the star-nosed mole and lantern fly. Since they are both animals, they belong to the natural world and are thus natural, not fantastic.

… Of course, to name them ‘fantastic’ is a misuse of language, but a significant one. In any event, being subject myself — perhaps unwittingly — to the diffuse pressure exerted by language, I was induced to launch the idea (surprising, to say the least, especially to me) of the natural fantastic. I first used the term in connection with an insect from north-east Brazil, the lantern fly, and a North American mammal, the star-nosed mole or Condylura. These two animals’ appearance made me resort to a category whose specious nature I could easily perceive. Quite obviously, these creatures were not fantastic because they were a part of nature. Just as obviously, they seemed fantastic, and even gave quite an exceptional sense of the fantastic: the tree-dwelling homopteron, on account of its frontal protuberance, which is almost as big as its body and deceptively suggests a crocodile’s muzzle; and the subterranean vertebrate, on account of its snout, which sports a crown of twenty-two short tentacles of live pink flesh, all mobile, sensitive, and retractable, flaccid or tensed at will, and very vaguely like an intricate starfish or some horrible corolla. (emphasis mine) The Edge of Surrealism: A Roger Caillois Reader by Caillois via [1]